pH Swing Method Research Articles

1,2-bis(triethoxysilyl)methane (BTESM)-derived organic–inorganic membranes with controlled sol particle size: Preparation and pervaporation dehydration performance of isopropanol aqueous solutions

1,2-bis(triethoxysilyl)methane (BTESM) is a bridged-type organoalkoxysilane, which was utilized for organic–inorganic membranes preparation via a sol–gel method to achieve high permselectivity, thus are promising for dehydration and purification of organic solvents. However, the particle size of the derived sol is crucial for membrane preparation during sol–gel processing, significantly influencing the membrane’s performance. Herein, we reported a novel strategy of controlling the sol particle size by adjusting the pH during the preparation process. BTESM-derived membranes were prepared by BTESM-H+ sol and BTESM-swing sol, which were subsequently employed for the dehydration of isopropanol (IPA) through pervaporation (PV). Compared with the BTESM-H+ sol, the particle size of the BTESM-swing sol prepared by the pH-swing method was increased from 1.3 nm to 32.7 nm. The larger particle size could hinder the permeation into the intermediate layer. Meanwhile, the dense arrangement of silica particles could efficiently retain IPA molecules, thereby substantially enhancing the rejection rate of IPA. The average particle size of the BTESM-swing sol was 18.7 nm at an acid-base ratio of 1.2 and acid-silicon ratio of 0.55, resulting a permeate flux of 1.04 kg·m−2·h−1 and separation factor of 521, while IPA rejection rate was 97.84 %. This work has shown that adjusting the pH during the process is a useful strategy of controlling the sol size for the preparation of organic–inorganic membranes. Hence, the control of sol particle size to achieve a high-performing membrane layer offers a novel approach to producing diverse membrane materials.

Renewable ammonium chloride-induced calcium leaching of gold and copper tailings and selective preparation of vaterite CaCO3 by CO2 mineralization

This study introduced a high-value green processing solution for copper tailings to prepare high-purity vaterite from gold-copper tailings. The active reaction components were determined through Gibbs free energy calculations. The study discusses the impact of different experimental parameters on the leaching rate of Ca2+ and the degree of mineralization in gold-copper tailings, and then considers the influence of different dispersants on improving the stability of vaterite. The results show that the optimal leaching efficiency of Ca2+ in the NH4Cl solution is 75%, and the optimal mineralization efficiency is 87%. Under appropriate mineralization conditions, vaterite CaCO3 with d50 < 12 μm, a purity of 98%, and a whiteness of 98.7% can be prepared. The precipitated vaterite is affected by the pH swing method connected with the neutralization reaction. The presence of excess NH3 and supersaturated CO2 are necessary for the formation of vaterite-type CaCO3. Morphological analysis results show that dispersants such as polyacrylamide and sodium tripolyphosphate can improve the dispersibility of vaterite, preventing agglomeration and enabling its stable existence in aqueous solutions. Nanoparticle aggregation and crystal growth are two possible mechanisms for the formation of vaterite. The leaching-mineralization cycle technology using NH4Cl as the leaching solution meets the requirements for CO2 removal and alkaline industrial waste residue treatment, while avoiding the significant consumption of chemical reagents in traditional mineralization. This method provides a pathway for producing vaterite using copper tailings and has a broad market prospect.

Synthesis of amorphous silica-alumina with enhanced specific surface area and acidity by pH-swing method and its catalytic activity in cumene cracking

High-performance amorphous silica-alumina (ASA) is an important acid material in the chemical industry. In this work, we successfully synthesized mesoporous ASA with enhanced specific surface area, Lewis acidity and total acidity by pH-swing method. The morphology and structure of ASA were systematically investigated. With the increase of SiO2 content from 20.1% to 67.0%, the morphology of ASA varied considerably from fibrous to granular, and the specific surface area decreased from 436 to 306 m2 g−1. Meanwhile, the gradual increase of surface silanol groups facilitated the formation of pseudo-bridged silanol groups and promoted the formation of Brønsted acid sites (BAS). Furthermore, the concentration of BAS was found to increase with increasing tetra- and penta-coordinated aluminum and decrease with increasing octa-coordinated aluminum. In addition, the acidity-activity relationship was also explored by applying the as-synthesized ASA samples directly to the cracking reaction of cumene. The results showed that the activity of ASA was proportional to the concentration of BAS. The pH-swing method can controllably synthesize ASA materials with different structures and acidity for petrochemical industry.

Understanding the acid dissolution of Serpentinites (Tailings and waste rock) for use in indirect mineral carbonation

Solid carbonates can be formed by indirect mineral carbonation using the pH swing method for CO2 storage. It occurs at low temperatures and pressures in four stages: acid dissolution, purification, carbonation, and recovery. Although acid dissolution requires short reaction time that favors the extraction of reactive metals, lack of knowledge about the effects of temperature, acid concentration and particle size on metal extraction reveals that it is a critical stage of indirect mineral carbonation. In order to employ mining waste for indirect mineral carbonation, acid dissolution of serpentinites, tailings (SERP-GO) and waste rock (SERP-MG) were investigated, and their performances were compared. A Taguchi experiment design was used to assess the parameters that influence the acid dissolution process. Hydrochloric acid was used in acid dissolution for Mg and Fe extraction from both, SERP-GO and SERP-MG, and the process parameters (temperature, acid concentration, particle size and excess acid) were evaluated for 2 h of reaction. According to signal-to-noise ratio analysis of the Taguchi method, the optimized condition for Mg extraction of both samples was at 70 °C, 4 M HCl, 69 μm and twice the excess acid. The results indicated that of Mg and Fe extraction from de samples was more efficient for SERP-GO (71% and 85%) than SERP-MG (33% and 31%). Based on these findings, it was possible to have a better understanding of the factors affecting the dissolution of serpentinite tailings, to develop a more effective process for the use of mining waste in the indirect mineral carbonation process.

Synthesis of Amorphous Silica-Alumina with Enhanced Specific Surface Area and Acidity by Ph-Swing Method and its Catalytic Activity in Cumene Cracking

Synthesis of Amorphous Silica-Alumina with Enhanced Specific Surface Area and Acidity by Ph-Swing Method and its Catalytic Activity in Cumene Cracking

Purification of landfill gas by extracted calcium ions from municipal solid waste incineration fly ash

This study proposes the utilization of CO2 based on the purification of landfill gas (LFG). The process included absorption of CO2 from LFG using monoethanolamine (MEA) absorbent, extraction of calcium ions from municipal solid waste incineration (MSWI) fly ash using various acids, and formation of calcium carbonate using the extracted calcium ions. During LFG purification, the concentration of CH4 in the gas after absorption was time dependent. The pH swing method was used for the extraction of calcium ions and comprised three phases: calcium ion leaching from MSWI fly ash phase, removal of cations from the supernatant, and calcium ion recovery. Hydrochloric and nitric acids, known as strong acids, and citric, acetic, and formic acids, which are weak acids, were used as extraction agents. Hydrochloric acid, nitric acid, acetic acid, and formic acid showed significant calcium ion recovery rates of 99.32%, 99.18%, 98.35%, and 97.99%, respectively, whereas citric acid showed a relatively low recovery rate of 82.82%. The extracted calcium ions were converted into calcium carbonate by reacting with ionic CO2 in the saturated MEA. The calcium carbonate formed showed different crystal structures based on the extraction agent used: aragonite for hydrochloric acid and nitric acid, amorphous CaCO3 for citric acid, vaterite for acetic acid, and calcite for formic acid. The results of this study can be applied to various CO2 utilization processes based on LFG and MSWI fly ash.

Advanced pseudopolymorph control of magnesium carbonates using structural properties of amines for CO2 utilization based on post-treatment of desalinated brine

Herein, we proposed a novel method for controlling magnesium carbonate pseudopolymorphs as part of the development of post-treatment technology for desalinated brine and CO2 from industrial plants. Mg(OH)2 was separated from reject brine using the pH swing method, and CO2 delivery into Mg(OH)2 was conducted by amine type CO2 carriers, including alkanolamine, alkylamine, and multi-amines with numerous amino groups, by utilizing the effects of their structural properties on pseudopolymorph control. The products formed by reaction between Mg(OH)2 and CO2-saturated amine carriers were analyzed by X-ray diffractometry, Fourier-transform infrared spectroscopy, and scanning electron microscopy for verification of crystal properties and thermogravimetry for purity analysis. The results demonstrated that through use of appropriate amine type CO2 carriers, particular magnesium carbonates can be formed. In conclusion, we proposed a novel process for pseudopolymorph control of magnesium carbonates without the need for excessive changes in reaction conditions and additional transformation inhibitors. With further optimization for scale-up and the process design of a carbon capture and utilization (CCU) plant, the proposed method can contribute to the commercialization of CCU processes where particular types of magnesium carbonates are targeted. Further, post-treatment of desalinated brine, efficient mitigation and utilization of CO2 will be achieved simultaneously.

Carbon utilization based on post-treatment of desalinated reject brine and effect of structural properties of amines for CaCO3 polymorphs control

This study proposed simultaneous disposal and utilization process of desalinated reject brine and CO2, or brine-based carbon capture and utilization (CCU). Using metal cations in desalinated reject brine, the feasibility of amines with different structural properties as crystallization inhibitors, morphology controller of calcium carbonate formation was investigated in this study. Ca2+ ions were separated from desalinated reject brine in a salt refining plant by using a pH swing method and used for CaCO3 formation. Through this process, disposal of the large amount of metal cations in the desalinated reject brine was achieved. CO2 was selectively captured by alkanolamine, alkylamine, and multi-amine CO2 carriers with numerous amino groups and carried into the separated Ca(OH)2 to generate CaCO3. X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FT-IR) were used to investigate crystal shape and polymorph of the product. Additionally, thermogravimetric/derivative thermogravimetric analysis (TG/DTG) was used for purity analysis. The results demonstrated that crystallization inhibition depending on structural properties of amines was possible, leading to successful CaCO3 polymorph control. The results of this research can be applied to process where the industrial production of metal carbonates based on inorganic CCU and disposal of the desalinated reject brine were involved.

Hydrometallurgical recycling of surface-coated metals from automobile-discarded ABS plastic waste

The ammoniacal leaching of surface-coated metals from automobile-discarded ABS plastics followed by their recovery through solvent extraction has been investigated. The leaching of ABS (typically containing 4.1% Cu, 1.3% Ni, and 0.03% Cr) could efficiently dissolve the ammine complexes of Cu and Ni, leaving Cr unleached as fine particles. The optimization studies for achieving the maximum efficiency revealed that the leaching of metal ions in different ammoniacal solutions follows the order CO32− > Cl− > SO42−. The leaching carried out in a carbonate medium by maintaining the total NH3 concentration 5.0 M at a NH4OH/(NH4)2CO3 ratio of 4:1, pulp density of 200 g/L, agitation speed of 400 rpm, temperature of 20 °C, and time of 120 min yielded the optimum efficiency of >99% Cu and Ni (i.e., 8.14 g/L and 2.57 g/L, respectively, in the leach liquor). Subsequently, the solvent extraction of metals from ammoniacal leach liquor as a function of extractant (LIX 84-I) concentration and organic-to-aqueous (O:A) phase ratio was examined. Based on the extraction data, a three-stage counter-current extraction at O:A = 1:1 was validated using 0.8 M LIX 84-I, yielding the quantitative extraction of both metals into the organic phase. Thereafter, the stripping of metals in acid solutions indicated that 0.5 M H2SO4 could quantitatively strip Ni from the loaded organic phase; however, ∼27% Cu was also co-stripped. The rest of Cu from the Ni-depleted organic phase was separately stripped with 1.0 M H2SO4 that can be directly sent to the electrowinning process. On the other hand, the co-stripped metals from the acidic solution can be easily separated, again using LIX 84-I as the extractant, by adopting the pH-swing method. Finally, a process has been proposed for the hydrometallurgical recovery of surface-coated metals from waste ABS plastics; that does not affect the physicochemical characteristics of the polymer substances for their reuse.

Experiment and kinetic modeling for leaching of blast furnace slag using ligand

Abstract Blast furnace (BF) slag is recognized as an excellent source of precipitated calcium carbonate (PCC) and CO2 sequestrating agents because it contains a significant amount of calcium. The pH-swing method is a promising process for commercialization of the slag carbonation process among the other indirect carbonation methods due to the high selectivity of calcium extraction and production of high quality PCC. In this study, ligand-based solvents, such as acetic acid, gluconic acid, and citric acid, were examined as new leachates for the extraction step in the pH-swing process. Citric acid showed the highest efficiency, and the extraction efficiencies were estimated based on particle size distribution, concentration of solvent, solid-to-liquid ratio, stirring speed, and temperature. The calcium extraction rate increased with decreasing particle size and S/L ratio and with increasing concentration of solvent and stirring speed. However, extraction efficiency decreased significantly at high temperature because calcium-citrate complexes were precipitated. Along with the experiments, a kinetic study based on the shrinking core model was carried out at temperatures lower than 30 °C. Our results demonstrated that the extraction process was controlled by a surface chemical reaction, and the activation energy was estimated to be 50.74 kJ mol−1.

Producing vaterite by CO2 sequestration in the waste solution of chemical treatment of recycled concrete aggregates

A recent study showed that the properties of recycled concrete aggregates (RCAs) can be significantly improved by acetic acid treatment. This study demonstrates that a useful product, vaterite, a metastable polymorph of calcium carbonate, can be produced from the waste solution of this treatment through an improved pH swing method. The salient feature of this improved method is that ethanol is added into the waste solution to not only prohibit the phase transformation of vaterite, but also to suppress the ionization of the regenerated acetic acid. This method allows not only more vaterite to be produced, but also more acetic acid to be regenerated from the waste solution through a CO2 bubbling method. Meanwhile, CO2 can be permanently stored in the produced vaterite to achieve sustainable CO2 sequestration. Experimental results confirm that near-pure vaterite can be produced from the waste solution by adding 20% ethanol into the waste solution. 1 kg RCAs can be used to produce about 76.6 g vaterite and permanently store 33.7 g CO2 by using this new method. The potential application of the produced vaterite as an internal curing agent for high performance concrete has also been demonstrated.

Leaching optimization of mining wastes with lizardite and brucite contents for use in indirect mineral carbonation through the pH swing method

This study investigated the leaching process in order to maximize Mg and Fe extraction and to produce amorphous silica (SiO2) with high purity. For this, a mining waste identified as S-GO was employed; which is a serpentinite rock with high lizardite 1T and native brucite contents. A Taguchi Experiment Design was used in order to assess the parameters that influence the leaching process such as: granulometry, hydrochloric acid concentration (HCl), leaching temperature, and mass/volume ratio. Furthermore, thermogravimetric analysis (TGA) was done to understand the interrelation between the mineral structure and leaching performance. Results show that lizardite 1T-bearing serpentinite presents a low content of tetrahedral Al3+ and high octahedral Fe3+ contents on S-GO. Native brucite delayed the formation of a hydrated silica layer and improved dissolution of serpentines. For this, Mg and Fe extractions are efficient, reaching 88 ± 2% of Mg and Fe extracted during the first 30 min of reaction, under mild process conditions: stoichiometric mass/volume ratio, 1M HCl concentration, pressure of 1 bar, temperature of 100 °C, and 300 μm particle size. On the other hand, an excess of acid improves Mg and Fe extraction by only 10 ± 5% for S-GO. Such characteristics reduce energetic penalties and costs involved on indirect mineral carbonation processes by the pH swing method.

Network engineering of a BTESE membrane for improved gas performance via a novel pH-swing method

Organosilica microporous membranes were fabricated from 1, 2-bis (triethoxysilyl) ethane (BTESE)-derived sols prepared in acidic pH via the pH-swing method. This method includes two steps whereby a specific amount of NH3 was added into the acid sols and switched to acid after a reaction of several minutes. We found that the size of the BTESE-derived sols by pH-swing could be controlled via the H2O/BTESE molar ratio and the reaction time in alkali. Under the same H2O/BTESE ratio of 60, the BTESE-derived sols prepared in the pH-swing method showed an increase sol size in contrast with the acid method, and the sol size was easily controlled by the dominating reaction in alkali pH – the condensation reaction. Gas permeation results showed that some gases (He, H2, N2, C3H8, SF6) permeated the membrane that was prepared using the pH-swing sols (pH-swing membrane) at approximately twice the rate shown by the membrane prepared using acid sols (acid membrane); H2 permeance levels of the pH-swing membrane and the acid membrane were 3.4×10−6 and 1.6×10−6molm−2s−1Pa−1 at 200°C, respectively. The pH-swing membrane also maintained similar H2/C3H8 permeance ratios of ranging from 2600~5800, confirming that pH-swing processing is an innovative method for improvement in the gas permeance of BTESE-derived organosilica membranes. One possible reason for these results could be that the membranes prepared using the pH-swing sols increased the size of the sols, which reduced the sol penetration into the intermediate layer. Moreover, the high cross-linking that was caused by pH-swing increased the thermostability of the BTESE-derived organosilica networks. The CO2/CH4 and CO2/N2 permeance ratios for the pH-swing membrane were as high as 90 and 28, respectively, at 50°C.

ChemInform Abstract: Carbon Dioxide Mineral Carbonation Through PH‐Swing Process: A Review

AbstractReview: 15 refs.

Carbon Dioxide Mineral Carbonation Through pH-swing Process: A Review

The promotion of carbon dioxide (CO2) reduction methods is due to the fact that the CO2 concentration has been increasing rapidly in 21st century. In order to prevent further damage of the environment caused by greenhouse gases, CO2 concentration should be stabilized by increasing CO2 fixation, which can reduce CO2 emission into the atmosphere. Mineral carbon sequestration or mineral carbonation is the process of utilizing minerals, mostly rich in calcium and magnesium (Ca/Mg) as the feedstock in reaction with CO2 to produce stable solid carbonates. Mineral carbonation through indirect pH swing process is a very effective method for producing calcium and magnesium carbonates. The Ca/Mg ions are extracted out of feedstock using suitable solvents at low pH condition and then in the second step the leached Ca/Mg ions are carbonated at elevated pH condition. In this paper the state-of-the-art of the carbonation involving pH swing method is updated.

Mineral carbonation of PGM mine tailings for CO2 storage in South Africa: A case study

The ultra-fine milled tailings generated during the processing of PGM ores in South Africa have a theoretical potential to sequester significant amounts of CO2 (∼14Mt per annum) through mineral carbonation. Mg-bearing orthopyroxene is the major sequestrable mineral in these tailings, which also contains significant quantities of Ca-bearing plagioclase, as well as minor quantities of clinopyroxene, olivine, serpentine and hornblende. In this study, the feasibility of using PGM tailings to sequester CO2 has been investigated empirically using the two-step, pH swing method. The rates and extents of cation (Ca, Mg and Fe) extraction and subsequent carbonation were determined and compared. Both organic (oxalic and EDTA) and HCl solutions were utilised in the cation extraction step, which was conducted at time periods up to 8h and at a temperature of 70°C. The extents of cation dissolution were relatively low under all experimental conditions investigated, particularly for the case of Mg (between 3.3% and 5.0% extraction). A comparison of the extents of leaching with the mineralogical composition of the tailings indicated that the extracted Mg originated primarily from clinopyroxene, with the orthopyroxene remaining relatively inert under the experimental conditions. Subsequent carbonation of the acid leach solution after pH adjustment with NaOH resulted in the rapid formation of a number of carbonate minerals, including gaylussite (Na2Ca(CO3)2·5(H2O)), magnesite (MgCO3), hydromagnesite (Mg5(CO3)4(OH)2·4H2O), dolomite (CaMg(CO3)2), ankerite (Ca(Fe,Mg)(CO3)2), and siderite (FeCO3). On the basis of these findings, further studies will be focused on developing a better understanding of the factors affecting the dissolution of Mg-bearing orthopyroxene minerals, and on exploring alternative leach reagents and conditions, with a view to developing a more effective process for the accelerated carbonation of PGM tailings.

Preparation and Evaluation of the Composite Containing USL Zeolite-Supported NiW Catalysts for Hydrotreating of FCC Diesel

Two different ways, including an in situ synthetic method and a mechanical mixing method, were used to combine zeolite USL (ultra stable L) with alumina for preparation of a new composite support material of hydrotreating catalyst. The physicochemical properties of samples were characterized by means of XRD, N2 physisorption, SEM, FT-IR, 27Al MAS NMR, NH3-TPD, H2-TPR, and UV−vis DRS. Composite supports containing different contents of zeolite USL and Al2O3 were prepared by in situ synthetic method based on a modified pH-swing method, which showed a higher specific surface area, pore volume, as well as average pore diameter compared with the supports prepared via a mechanical mixing method. Corresponding NiW/γ-Al2O3−USL series catalysts were obtained by the incipient-wetness impregnation method, and the activities of these catalysts for FCC diesel hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) were evaluated in a high-pressure microreactor system. The assessment results indicated that the catalyst with 10 m% USL in the support prepared by the in situ method showed the highest HDS and HDN conversions, which reached a maximum of 99.3% and 94.1% for HDS and HDN, respectively. In addition, the swing pH method plays an important role in preparation of the Al2O3 support, and NiW/γ-Al2O3(P) (prepared by the swing pH method) catalyst also gave better performance for the HDS and HDN of diesel oil. These activities were much higher than those over a kind of industrial catalyst of RN10 and were also better than the corresponding catalyst in which the support was obtained by the mechanical mixing method.

Ultrasonic-assisted pH Swing Method for the Synthesis of Highly Efficient TiO2 Nano-size Photocatalysts

A new route for the preparation of nanocrystalline TiO2 particles based on the pH swing method assisted by ultrasonic irradiation in the presence of a surfactant (Pluronic P-123) has been successfully achieved. The prepared TiO2 catalysts were calcined from 400 to 800 °C and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Fourier transformed infra-red spectroscopy (FTIR), gas adsorption measurements (BET) and thermogravimetirc measurements (TAG/DTA) analyses. Characterization results revealed that the enhancement in the particle size of TiO2 by the pH swing method could be controlled by combining the pH swing with ultrasonic irradiation. Increasing the calcination temperatures led to an increase in both the particle and pore size, whereas the surface area and pore volume gradually decreased. A synergistic effect was observed in the combined process of pH swing with ultrasonication, yielding small TiO2 particles as well as high surface area, pore volume, pore diameter, and crystalline anatase phase. The activity of the catalysts was investigated for the oxidation of 4-chlorophenol (4-CP). TiO2 prepared with 15 times pH swing and calcined at 700 °C was found to show the highest rate for the oxidative degradation of 4-CP when compared to the TiO2 sample prepared with just 1 time pH swing and to the commercial P-25 TiO2 Degussa photocatalyst. Thus, a novel approach in controlling the various physico-chemical parameters of TiO2 nanoparticles was developed.

Preparation of alumina–titania nanofibers by a pH-swing method

Fibrillar Al 2O 3–TiO 2 were obtained by a pH-swing method by varying the pH from 2 to 8, and repeating this range (e.g. a cycle) several times, in combination with different ways to incorporate titanium oxisulfate. Depending on the stage in which Ti-oxisulfate is added to the alumina, large anatase aggregates or very dispersed TiO 2 domains were obtained on/in alumina nanofibers. Thus, when using a co-precipitation method in combination with pH-swing between pH 2 and 8, with six pH cycles, a high TiO 2 dispersion can be obtained, as indicated by Raman spectroscopy and X-ray diffraction. On the other hand, when adding Ti-oxysulfate at the end of the pH-swing procedure, though alumina nanofibers were formed, anatase was predominantly located as large particles in the interfibrillar voids. Specific surface area and total pore volume of nanofibrillar Al 2O 3–TiO 2 were around 300 m 2/g and 1 cm 3/g, respectively, after calcining at 500 °C. The highly disordered intertwined fibrils favored high pore volume values, even upon calcinations at 500 °C.

Preparation of TiO2 nano-particle photocatalysts by a multi-gelation method: the effect of pH change

TiO2 photocatalysts were prepared by a multi-gelation method and the effect of the changes in the pH during the pH swing times, i.e., by a controlled pH swing, on the morphology of the TiO2 particles was investigated. The photocatalytic properties of the TiO2 catalysts prepared by controlled pH swing were compared with TiO2 particles prepared without adjusting the pH during the swing times. The photocatalytic degradation reaction of these TiO2 catalysts was investigated by comparing their effectiveness in 2-propanol oxidation. The experimental results showed that the TiO2 photocatalysts prepared without adjusting the pH performed better in controlling the important parameters of the catalysts such as particle size, surface area, anatase/rutile phase ratio and pore size, as well as pore volume than the TiO2 photocatalysts prepared by a controlled pH swing method.